Hollow substrates with a large aspect ratio are commonly used in various technological fields such as catheters or endoscopes for medical materials and packaging for food or pharmaceutical applications. The expression “large aspect ratio” means that the hollow substrate has at least one dimension that is much larger than another one, and more particularly that the length of the substrate is much larger than a dimension of the substrate aperture. FIG. 4 shows two examples of hollow substrates which have a large aspect ratio L/a, where L is the length of the substrate and a is the smallest dimension of the substrate aperture. A tube 1 comprises an inner cavity having a large length L with respect to the diameter a of the tube. Large aspect ratio substrates may also have a form of a flat box 101 which exhibits a little aperture height a in comparison with its length L.
A general difficulty of the use of plasma treatment is the complexity to treat internal walls of such substrates. Indeed, plasma treatment of this kind of substrate is difficult to perform since the plasma creation into large aspect ratio hollow substrates generally presents a lack of plasma uniformity and thus of treatment. A uniform treatment is ensured only if, for the whole substrate length, the gas precursor concentration, the local plasma density and the pressure are rigorously constant.
The creation of the plasma inside the substrate is carried out by applying electrical energy to the process gas. The electrons are accelerated by an electric field and ions are created from inelastic collisions between gas molecules and the accelerated electrons. The electrical energy to accelerate electrons in the gas is generally performed by a varying electric field, a varying magnetic field, or both.
Two main problems occur when the aim is to treat or to deposit plasma along a hollow substrate. The first problem concerns the difficulty to create a uniform plasma density along the substrate length. Indeed, to achieve this condition, a constant electrical energy must be applied to the substrate, which becomes less feasible over a certain size.
The second problem is that uniformity of treatment along the whole length can be ensured only if a constant quantity of precursor reacts all along the substrate length. Even if a special energy source arrangement can be implemented to create uniform plasma density along the substrate length, the precursor concentration will decrease irremediably as soon as the gas precursor has flowed the substrate, since higher precursor consumption will occur at the substrate gas inlet.
To remedy this problem, several solutions have been developed. One of them, described in U.S. Pat. No. 4,692,347 and illustrated in FIG. 5, consists of shifting a tube 502 to be treated with respect to a fixed plasma source 505 in a vacuum chamber 501. The tube 502 to be coated is initially wound on a reel 508 with an extremity in communication with a monomer source 503 via a flow controller 504. The plasma is created inside the tube 502 by continuously passing the tube in a glow discharge zone formed by the fixed reactance coupling source 505 formed from two electrodes radio frequency powered. The tube part whose interior wall has been coated is wound on a receiving reel 509. A low absolute pressure is maintained inside the tube by evacuating means 506 and 507 connected to the other extremity of the tube.
However, such a solution has drawbacks. To roll and unroll thin tubes of low or high stiffness can lead to local shrinking or folding, that is to say irreversible tube deformations. Moreover, the structure and implementation of the evacuating means are complex and it is difficult to guarantee a good pressure control along the tube. These difficulties affect not only the reliability of the plasma treatment of a tube but also the costs and the rapidity of the treatment.
Conversely, another solution is to attach the plasma source to a motion mechanism for shifting the plasma source with respect to the tube to be treated. However, such a mechanism is complex and does not permit to control the parameters for a plasma uniform treatment in the substrate. The velocity and the precision of the plasma source motion required for uniform plasma treatment leads to develop electronic control system sensibly increasing the cost of the treatment. Moreover, such a device is limited to single tube treatment.